Modulation of osteoclast numbers has a profound influence on osteoclast-mediated bone degradation in both pathological bone loss and during normal bone metabolism. TGF-li is abundant in the bone environment and has been implicated in regulation of osteoclast differentiation. We have discovered TIEG1 (TIEG), a transcription factor whose expression is rapidly increased in human osteoblasts following TGF-R treatment. To better understand TIEG's role(s) in bone metabolism, we have generated mice lacking TIEG. Bones from these mice are weaker and smaller with increased trabecular spacing in the femoral head compared to age matched wildtype littermates. Marrow- and spleen-derived osteoclast precursors from the TIEG-/- mice have an amplified ability to differentiate into osteoclasts in vitro and enhanced NFKB activation. Moreover, TGF-fl treatment during differentiation did not stimulate differentiation, unlike osteoclast precursors from wildtype mice. Calvarial-derived TIEG-/- osteoblasts have a reduced capactiy to support osteoclast differentiation, in part due to increased OPG and decreased RANKL expression. These observations and the published literature lead to our central hypothesis that TIEG expression in both osteoclast precursors and osteoblastic support cells is important in modulating osteoclast differentiation and that osteoclast precursor TIEG expression is essential for TGF-li stimulation of differentiation. To test this hypothesis, the Specific Aims of this proposal are to 1. Elucidate the mechanisms by which NFKB pathway activation is enhanced in TIEG-/- osteoclast precursors. 2. Resolve the role of enhanced NFKB signaling in the increased differentiation of TIEG-/- osteoclast precursors. 3. Determine the roles of TIEG in TGF-li stimulation of osteoclast precursor differentiation in vitro. 4. Discover the molecular mechanisms involved in the increased expression of OPG mRNA and protein in TIEG-/- calvarial cells cultured in vitro. 5. Ascertain the mechanism of TIEG stimulation of RANKL gene expression in osteoblasts. Since the rate of bone loss is mainly determined by the number of osteoclasts, understanding regulation of osteoclast differentiation is likely to provide important avenues for therapies to slow bone degradation during pathological bone loss. These studies will add to this knowledge and increase our understanding of osteoclast-mediated bone loss. Further, these studies should provide key information on the molecular mechanisms of osteoclast differentiation.